1. Field of the Invention
The present invention relates to a light-emitting device including a light-emitting element mounted in a ceramic package.
2. Description of the Related Art
There is known a light-emitting device including a light-emitting element (also referred to as “LED chip”) disposed in a package. The package is made of synthetic resin or ceramics. The ceramics is advantageous in that it has a lower thermal resistance than the synthetic resin and can efficiently release the heat of the LED chip.
Light-emitting devices employing a ceramic package are generally of a structure wherein an LED chip disposed in the mounting space in the ceramic package is encased in molded synthetic resin. For mounting as large an LED chip as possible in the ceramic package or mounting as many LED chips as possible in the ceramic package, i.e., for obtaining as large an effective mounting area in the ceramic package as possible, it is desirable to provide a rectangular mounting surface in the ceramic package. The reason for such a rectangular mounting surface is that if an LED chip which is a rectangular planar shape is mounted on a non-rectangular mounting surface such as a circular or elliptical mounting surface, then a large dead space is created in the ceramic package and the effective mounting area is reduced in the ceramic package.
Heretofore, it has been customary to form as large an electrode pad as possible on a rectangular mounting surface on the bottom of the mounting space, and to place an LED chip on the electrode chip. If a single electrode pad is to be formed, then it is formed on the entire mounting surface. If a plurality of electrode pads are to be formed, then, as shown in FIG. 2 of the accompanying drawings, electrode pads 50 are formed with necessary minimum clearances provided therebetween, and LED chips 51 are disposed on the selected electrode pad from among electrode pads 50. The clearance provided between electrode pads 50 serve to provide electric insulation. In either case, it can be seen from FIG. 2 that the rectangular mounting surface has four corners covered with electrode pads 50.
However, if the mounting surface (mounting space) is rectangular in shape, then when the molded synthetic resin that fills the mounting space expands and contracts as the LED chips are heated and then cooled, stress that has developed in the molded synthetic resin concentrates on the corners of the mounting space, tending to cause the molded synthetic resin to peel off.
JP-A No. 2004-111937 discloses a light-emitting device wherein a ceramic base is exposed around a light reflector for preventing molded synthetic resin that fills a cavity in a ceramic package from peeling off from the light reflector on an inner wall surface of the cavity. In the disclosed light-emitting device, both the light reflector and the ceramic base are covered with the molded synthetic resin.
The cavity in the ceramic package disclosed in JP-A No. 2004-111937 is of an essential oval shape and, as a mounting space, has a non-rectangular bottom serving as a mounting surface. This is because the invention disclosed in JP-A No. 2004-111937 is not concerned with a technology for solving the problem of molded synthetic resin peel-off that is caused when stress that develops as the molded synthetic resin expands and contracts concentrates on particular spots in the mounting space. Consequently, the invention disclosed in JP-A No. 2004-111937 is unable to avoid a peel-off of the molded synthetic resin at the corners of the mounting space which is caused if the mounting surface is rectangular in shape and electrode pads are formed in as wide a range as possible on the mounting surface.
Therefore, there is a trade-off between increasing the mounting area and peel-off of the molded synthetic resin. If priority is given to increasing the size of the mounting area, then the molded synthetic resin becomes liable to peel off, and if the molded synthetic resin is not to peel off, then the mounting surface needs to have a shape that is free of corners, such as a circular or elliptical shape.